In domestic and commercial plumbing, it is sometimes necessary to install a shut-off valve to interrupt a flow of fluid in a pipe. It can also be advantageous to be able to install the valve without turning off the fluid supply for installation of the valve. A shear valve may be used for this purpose. Typically a shear valve has a cutting blade which is actuated across a pipe for shearing the pipe within the valve.
Shear valves sometimes fail to cut a pipe cleanly, because the frictional forces between the pipe and the blade are large. Furthermore, pipes sometimes collapse as they are being cut, because the blade acts downwardly from one side of the pipe. Thinning the cutting blade reduces the frictional forces, but the blade needs to be a certain size and thickness to have sufficient strength to make the cut. Once cut, the shear valve may not always provide a perfect seal for shutting off a fluid supply.
A further problem of a shear valve, is that once cut, the live pipe end can come out of the shear valve, due in part, to restricted flow causing back pressure on the pipe. A yet further problem of a typical shear valve, is that a threaded actuator for causing actuation of the cutting blade extends from the outer casing of the valve and is sealed on the thread. This can lead to leaks from the valve, because threads can be difficult to seal.
It is an object of the invention to provide a shear valve which reduces or obviates the aforementioned problems.